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The Solution for IP Service Quality

MARK VEIL

To justify the industry's recent scramble to offer IP-based services, one need only to examine the realities of the evolving public network:
Today's public network is experiencing a traffic growth rate that doubles every three to four months, while aggregate capacity is lagging by a conservative factor of five.

• A new generation of Internet-based applications, videoconferencing, distance-learning and multimedia services are beginning to mature and soon also will rely on the public network to reach customers.
• The public network has become a strategic, mission-critical component of enterprises' businesses. Such organizations rely on the public network not simply to compete but for their very survival.
• Data traffic volume has surpassed that of time division multiplexing (TDM) within the core of the public network. In fact, within five years, the volume of circuit-switched voice is expected to represent less than 10% of overall network traffic.

But in this quest to offer IP-based services, performance, quality and availability are suffering--even today's largest routers are stumbling under the weight. In order to accommodate this shift from an environment once dominated by TDM-based circuit-switching to one dominated by IP packet-based traffic, network architectures will have to change.

One of the more significant challenges facing carriers is migrating the existing TDM-based access network to one that is optimized for delivering packet-based IP services but still capable of delivering TDM-class voice services. Until recently, the only alternative for an operator planning to migrate to a packet infrastructure has been ATM. Although ATM provides the quality of service (QOS) necessary to support voice and data applications, its high cost and complexity were only justified for provisioning high-end data services or in the core of the network.

The telecom industry has arrived at a critical juncture--one of significant challenge but also one of great opportunity. Two new initiatives--multiprotocol label switching (MPLS) and differentiated services (DiffServ)--are emerging to provide network operators the tools to deliver sophisticated traffic engineering and management functionality, implement class and QOS, and provision secure data virtual private networks (VPNs) across IP backbones.

The DiffServ way

The DiffServ architecture--an Internet Engineering Task Force initiative--provides a way for routers within an IP network to give certain data sessions priority treatment according to "marks" assigned to packets in each data session. DiffServ, which can mark bits for up to 64 classes of service for use across WANs, provides functionality similar to that of MPLS, but it does so at Layer 3. DiffServ's strength lies in its ability to provide QOS functionality through its traffic management facilities, and it provides an excellent complement to MPLS for enabling end-to-end QOS for IP traffic.

Within the DiffServ architecture, the heaviest traffic classification and conditioning operations occur at the edge of the network, sparing an already overburdened core. Rather than encapsulating and labeling the packet, DiffServ uses the type-of-service field in IPv4 or the traffic class field in IPv6--renamed the "differentiated services code point"--for "marking" the packet's traffic classification. These markings serve to classify packets according to policies set forth in the traffic conditioning specification and will determine traffic flow placement and the per-hop behavior on DiffServ nodes along the path.

Additional traffic management services, such as metering and policing, occur at the ingress of the edge network. Traffic shaping occurs at the edge egress where the "marked" packets are aggregated into flows according to their classifiers.

The MPLS route

MPLS, another IETF initiative, integrates Layer 2 information about network links (bandwidth, latency, usage) into Layer 3 of an IP network to simplify and improve IP packet exchange. It provides network operators with a great deal of flexibility to divert and route traffic around link failures, congestion and bottlenecks.

When packets enter an MPLS-based network, they are given a label, also called an identifier. These labels contain information based on the routing table entry (for example, destination, bandwidth, delay and other metrics) and refer to the IP header field (source IP address), Layer 4 socket number information and differentiated service. Once this classification is complete and mapped, different packets are assigned to corresponding labeled switch paths.

The primary benefits of the MPLS architecture are its protocol independence, its ability to leverage QOS offerings and its sophisticated, domain-level traffic engineering facilities. Labeled switch paths behave in much the same manner as virtual circuits within ATM and frame relay domains but do so with much greater resource and performance efficiency. This allows network operators to engineer network traffic to minimize congestion, balance the network load and divert and route packets based on data-stream type and access customer.

Although the MPLS specification does not directly include tools for managing QOS, the intrinsic characteristics of the networkwide traffic flows for the labeled switch path provide a strong foundation for delivering QOS. By leveraging the rich set of traffic management services provided for within the DiffServ architecture, MPLS delivers QOS to the network layers where traffic management is required (Figure 1).

With this MPLS/DiffServ combination, network operators can prioritize traffic and provision network bandwidth to deliver toll-quality voice calls, truly differentiated data services and service level agreements (SLAs). Together, MPLS and DiffServ provide a scalable QOS solution for the core of the network.

QOS for multiservice networks

The current trends within the public network point to clear challenges for today's competitive carriers. Service providers must account for a network that, if not now, very soon will be dominated by packets and IP--from the customer premises to the network's core (Figure 2). If service providers are to ensure network performance, availability (meaning QOS) and scalability, they must have the ability to engineer and manage the traffic end-to-end. The current internetworking paradigm of the core public network, based upon the interior gateway protocol, can carry the industry no further. TDM and ATM-based access infrastructures add unnecessary and extremely burdensome overlay networks and are inefficient and ill-suited for an all-IP, packet-based world.

But as with every challenge, there is opportunity. New IP-based technologies for supporting integrated voice and broadband data services over a single link are providing competitive carriers the opportunity to aggressively attack new markets and meet the needs of the underserved small and medium-sized business markets. The latest generation of integrated, multiservice access products clearly is optimized for the new IP world order. However, to deliver a comprehensive and competitively priced set of services, carriers must be able to manage QOS. MPLS and DiffServ are ideally suited to meet these needs.

Currently, more than 90% of all service provider data revenue comes from leased line and frame relay services that provide QOS and absolute data security. Mission-critical applications that run on top of these connections will require equivalent QOS guarantees and privacy from a packet-based network. To meet these requirements, network operators must be able to guarantee bandwidth and latency by reserving resources through the WAN. MPLS, which can run over any link layer, provides a connection-oriented Layer 2 technology under the IP service to support explicit QOS guarantees, including bandwidth, latency and bit-error ratio.

An equally important challenge facing many carriers is the gradual migration from a TDM-based access network to one that is optimized for delivering packet-based IP services. Using MPLS and DiffServ, network operators can leverage and serve customers from their existing circuit-switched infrastructure while migrating to packet-switched voice technologies. This is accomplished today by having the packet-based access network incorporate voice access gateway functionality and interfacing with Class 5 switches using GR-303. The same packet-based access network can deliver voice over IP to an IP backbone under a SoftSwitch call control. Again, MPLS provides the optimal solution for meeting the connection-oriented and connectionless requirements of current and future applications.

The competitive advantage

As the effects of deregulation gain momentum and competition between service providers intensifies, emerging and incumbent carriers seek new infrastructure solutions as a source of competitive advantage. Most network operators find it easier to support existing services on a new data-centric infrastructure than to adapt the existing circuit-switched network to support IP traffic.

Service providers' biggest challenge remains integrating heterogeneous access and transport technologies (circuits, cells and packets) onto a single, cost-effective, multiservice, packet-based network that can match the QOS levels provided by circuit-switched technology. Current infrastructure technologies, such as TDM and ATM, are at a significant disadvantage in terms of their ability to efficiently and cost-effectively integrate with the IP-dominated networks of the future.

The emergence of MPLS and DiffServ is enabling new packet-based access solutions that can support and deliver toll-quality voice and data services. MPLS and DiffServ together enable network operators to maintain guaranteed levels of QOS for IP and legacy voice and data transmissions while providing tools for managing traffic, deploying and monitoring SLAs and provisioning secure data VPNs across IP backbones. Ultimately, MPLS and DiffServ will enable service providers to provision QOS across entire networks.

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